CD8+ T cells (TCD8+ ) play an important role in controlling virus infections. TCD8+ recognize peptides of 8 to 10 residues derived from viral proteins located in the cytosol of virus infected cells. These peptides are recognized in a complex with class I molecules encoded by the major histocompatibility complex (MHC). The MHC also encodes two molecules, termed TAP1 and TAP2, that combine to create a complex (TAP) that delivers peptides from the cytosol into the endoplasmic reticulum (ER) where they are assembled with class I molecules. In the absence of TAP, cells have a slight, but still significant, capacity to deliver cytosolic peptides to the ER. In this project we investigate whether this is due to the multi drug resistance protein (MDR), a transporter related to TAP that plays an important role in the resistance of tumors to chemotherapy.